Electrographic imaging and development processes have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of forming a latent electrostatic image on an insulating electrographic element, such as a photoconductive insulating layer coated on a conductive support. The electrostatic latent image is then rendered visible by a development step in which the charge image-bearing surface of the electrographic element is brought into contact with a suitable developer composition.
Liquid developer compositions of the type described, for example, in Metcalfe et al, U.S. Pat. No. 2,907,674, issued Aug. 6, 1959, have been used heretofore to develop latent electrostatic images. Such developers usually comprise a stable dispersion of charged particles known as toner particles comprising a pigment such as carbon black, generally associated with a resinous binder, such as, an alkyd resin, dispersed in an electrically insulating liquid which serves as a carrier. A charge control agent is sometimes included to stabilize the magnitude and polarity of the charge on the toner particles. In some cases, the binder itself serves as a charge control agent.
Liquid developers for electrography should possess both suspension stability and charge stability. They should also produce images that have (a) good wear resistance, (b) high image density, (c) high resolution, (d) low background, and (e) relatively low fusing temperatures. Various additives have been suggested to control or effect one or more of these desirable properties. However, it remains difficult to formulate liquid developers that achieve all or most of the desirable properties listed above.
Polyester resins have been suggested for use in liquid electrographic developers in the past. U.S. Pat. No. 4,052,325 to Santilli granted Oct. 4, 1977, discloses a highly useful electrographic developer composition containing a polyesterionomer. However, the wear or rub resistance of the images formed from this developer could be improved for some applications, such as those in which imaged surfaces are handled frequently.